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lime mortar allows slight movement without cracking the joints it also lets the wall breath and allows water vapour to escape out of the wall
plasticiser is to make the mortar more creamy and last longer fore use some types of sand will not hold water very well and a plasticiser is used to aid this to save knocking it up every five minutes lime will do this also depending on the sand. here in yorkshire the soft sand is mainley yellow that works well but you can get a red sand that is a pain
Thanks for the response Dave. What sort of construction (wall, driveway etc) would want a joint to be able to move slightly though?
such as in expanshon and contraction
Plasticier is an air entrainer, it makes bubbles in the mortar, so the mortar ends up like Aero chocolate bars.
It actually lightens the mix and make a it easier to work with, but porous.
Waterproof does the exact opposite , it drives out the air from the mortar , closing the mix up to reduce the passage of water through it, but making it heavier and harder to work with.
Don't use plasticier in foundations and retaining walls were you want to avoid the passage of water and definitely don't put plasticier and waterproof in the same mixes, as they cancel each other out by trying to produce different results.
Andy
You can get combined waterproofers and plasticisers that do a reasonable job.
Important also to know the difference between hydrated lime (used as a plasticiser) and hydraulic lime (used to make mortar when laying softer stones, especially in conservation work). Or even non hydraulic lime putty.
THe other reason washing up liquid is a poor plasticiser is that the bubbles created are too large.
Which is why you can you washing up liquid as a plasticier, though it's not recommended.
Beware, washing up liquid contains salt, which can wreck the integrity of mortar.
Lime, in itself, is plascticised and you shouldn't need any additives.
I working in a concrete lab for one of the largest concrete suppliers in the UK and have a city and guilds in concrete technology although it was many years ago. Here is my two pence worth.
Plasticiser is not an aerial entrainment additive there are specific products which do this. A plasticiser is a product that makes the concrete or morter more workable ( increases the slump) without adding water. The main advantage is also less water needs to be added and so the concrete or morter is stonger for the given slump. The old fairy liquid does the same thing but is liable to lower the strength of the mix rather than increase it.
Most people use it in morter for workability than strength and the main reason not to use is would largely be cost when using an OPC or PBF cement. I have no experience with lime so can't comment on that.
I think what you have described is known by those involved in landscaping as a wetting agent, such as those for getting water into the soil under lawns, which can be done with a bit of soap in the water, though again there are specific products made for the job.
The reason for using the minimum amount of water required to make the cement do it's job is that excess water will again effectively leave little voids, like bubbles, that weaken the concrete and make it more porous.
Years ago I worked on a site where there was a tremendous problem with water running through the mortar joints, the walls were built of hard impervious bricks and the water ran off them saturating the joints. Water ran straight through the single brick walls of the garages and left puddles on the floor, as well as running down inside the chimneys into the houses themselves. It cost the developer a lot of money with over twenty houses like it.
The mortar mix needs matching to the bricks and can't be filled with random addatives.
On another site the plasterers thought they could use ready mixed mortar for some internal rendering that had to be skimmed, that was another disaster, that required remedial work with a big hammer!
Andy
If you want chapter and verse, try this:
http://www.mortar.org.uk/documents/LT04-Admixtures.pdf